This application is based on Japanese Application No. 10-259780 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image reading apparatus that carries out image reading such as a color copy machine, a facsimile apparatus, a multiple function apparatus thereof, and a microfilm reader, and a chromatic aberration correction method applicable to such apparatuses. More particularly, the present invention relates to an image reading apparatus that corrects chromatic aberration caused by an optical system, and a method of correcting chromatic aberration that is applicable to such an image reader.
2. Description of the Related Art
Conventionally, an image reading apparatus that receives light directed to and reflected from a document original by a solid state image sensing device such as a CCD linear sensor through a lens to provide image data is known. In such an image reading apparatus, the light of the wavelength of red, (abbreviated as xe2x80x9cRxe2x80x9d hereinafter), green (abbreviated as xe2x80x9cGxe2x80x9d hereinafter) and blue (abbreviated as xe2x80x9cBxe2x80x9d hereinafter) are respectively received by a CCD linear sensor. Image data is output according to the received amount of light. However, the position where an image is formed on the CCD linear sensor by respective light of R, G, and B will differ from that of the other images due to the different index of refraction of each R, G, and B light to appear as chromatic aberration. When an image of a pattern with black vertical stripes on a white background is read, the position of the light of R, G, and B forming an image on the CCD linear sensor is deviated. As a result, color blur and the like is generated at the edge of the vertical stripe pattern.
To compensate for this effect of chromatic aberration, the conventional image reading apparatus corrected the output of the CCD linear sensor. For this correction, the R, G, and B outputs of a pixel of target and neighborhood pixels are read out. The outputs of the neighboring pixels of the target pixel are multiplied by a predetermined correction coefficient. The value that has small difference from that of the target pixel is taken as the value for the target pixel. This correction will be described in further detail.
Referring to FIG. 13, a CCD linear sensor 50 has a plurality of pixels arranged in the main scanning direction. The conventional chromatic aberration correction method is carried out according to the output of this CCD linear sensor. Here, the R, G, and B outputs of a target pixel 50A are R0, G0 and B0, and of a pixel 50B adjacent to target pixel 50A are R1, G1 and B1. The R, G, and B outputs of a pixel 50C adjacent to target pixel 50A are R2, G2 and B2. The correction amount of target pixel 50A is compared with a value which is the R, G, and B outputs of adjacent pixels 50B and 50C multiplied by a predetermined coefficient, and then corrected to a value of small color difference (R, G, and B output difference).
As specific examples of calculation, xe2x80x9cR=R1xc3x97N+R0xc3x97Mxe2x80x9d, xe2x80x9cB=B1xc3x97N+B0xc3x97Mxe2x80x9d, xe2x80x9cR=R2xc3x97N+R0xc3x97M, and xe2x80x9cB=B2xc3x97N+B0xc3x97Mxe2x80x9d are calculated for the G output G0 of target pixel 50A. M and N are constants determined by the property of the lens with the relationship of N+M=1. Respective combinations of the calculated R and B are compared with G0. The combination of R and B that has the smallest RGB difference is selected. The selected R and B are set as the corrected R output and B output of the target pixel.
The chromatic aberration correction of the conventional image reading apparatus is simply implemented by carrying out correction by image processing using only the information of the read out image data. No consideration was made whether the chromatic aberration was caused by the optical system or caused by color deviation of the document itself. Furthermore, there was no correction based upon detection of the chromatic aberration of the optical system.
In view of the foregoing, an object of the present invention is to provide an image reading apparatus that can properly read out a document image by detecting and correcting chromatic aberration caused by the optical system, and a chromatic aberration correction method.
According to an aspect of the present invention, an image reading apparatus includes a chromatic aberration correction chart with a high reflectance portion and a low reflectance portion, lower in reflectance than that of the high reflectance portion, a photoelectric conversion unit to receive light reflected from the chromatic aberration correction chart, and a correction processing unit correcting the chromatic aberration of image data according to a light receiving position in the photoelectric conversion unit receiving light of a first wavelength included in the reflected light from the chromatic aberration correction chart and a light receiving position in the photoelectric conversion unit receiving light of a second wavelength included in the reflected light from the chromatic aberration correction chart.
According to another aspect of the present invention, a chromatic aberration correction method of an image reading apparatus includes the steps of receiving light reflected from a chromatic aberration correction chart having a high reflectance portion and a low reflectance portion lower in reflectance than that of the high reflectance portion, detecting a first light receiving position in the photoelectric conversion unit receiving light of a first wavelength included in the light reflected from the high reflectance portion and a second light receiving position in the photoelectric conversion unit receiving light of a second wavelength included in the light reflected from the high reflectance portion, and correcting chromatic aberration of image data according to the first and second light receiving positions.
According to a further aspect of the present invention, a reading apparatus includes a light source, a correction chart having a portion formed in different density, an optical system focusing light from the light source reflected at the correction chart, a light receiving unit receiving light focused by the optical system, separated into light of predetermined wavelengths, and a processor correcting chromatic aberration caused by the optical system according to data of each separated light obtained by the light receiving unit.
According to the present invention, an image reading apparatus that can correct chromatic aberration accurately and that can read out a document original properly, and a chromatic aberration correction method are provided.